This invention relates to power supply circuits, and more particularly to a switching mode type of DC-to-DC converter having an improved method of current sensing.
Portable computers of the lap-top size usually have a built-in battery for operating from battery power alone, and also have an AC adapter, often outside the housing, for operating from line current. Two regulated power supplies are used; one is for operating off the battery, and converts +12 V dc to regulated +5 V and +12 V dc, and the other is for operating from 120 V ac line curent or for charging the battery, in which case both of the power supplies are operating if the computer is being used.
When a portable computer is operated on battery power, low power consumption becomes important so that the size and weight of the batteries required to provide the desired running time will be reduced. Also, since it is preferable to omit a fan in the design of battery powered equipment, high efficiency is needed to avoid high temperatures in power supply components. In my copending application Ser. No. 258,213 filled herewith, a highly efficient DC-to-DC converter is described to function as a regulated power supply for operating from the batteries in a portable computer.
For the AC to battery-voltage conversion, a combination of an unregulated power supply circuit to produce a high voltage DC level along with a switching mode rectifier circuit is an appropriate choice; the switching mode circuit provides DC-to-DC conversion with a reasonable degree of output voltage regulation. The regulated DC-to-DC converter such as that of the application Ser. No. 258213 is then used to supply the operating voltages for the computer, just as when only batteries are being used.
A type of controlled rectification device particularly well suited for use in a switching mode type of converter circuit is a power MOS transistor having a short, diffused channel and basically vertical current flow through the substrate; these devices are of reasonable cost, high efficiency (low forward resistance), fast switching, minimum cooling requirements, and offer a high impedance gate input so the drive circuitry can be of low cost logic-type circuitry rather than power-type or current-drive type of circuitry. For constructing this type of power supply circuit at low cost, it has become advantageous to use a commercially available integrated circuit device which performs the function of a pulse-width modulator to drive the gate of one of these MOS power transistors; the pulse width is controlled in response to load voltage, for example.
In controlling the pulses applied to the gate of a power transistor of the type used in these rectification circuits, some kind of filtering is usually employed to prevent or minimize adverse effects of current spikes caused by various transient conditions. By filtering out one category of undesirable voltage or current spike, however, another category may be unintentionally masked; for example, filtering to reduce unwanted effects of turn-on spikes caused by charging various circuit capacitances may have the undesirable result of allowing current spikes caused by transformer saturation to harm the power supply circuit.
It is therefore the principal object of this invention to provide an improved power supply circuit for use in a portable computer or the like. Another object is to provide a high-performance, low-cost, switching-mode type of power supply circuit, particularly one in which the method of current sensing is improved so that current spikes caused by transformer saturation are minimized. A further object is to provide a method of controlling the gates of power FETs in a switching mode type of power supply circuit to prevent undesirable current spikes. An additional object is to provide a power supply circuit for a small portable electronic device, particularly a low-cost circuit which will allow the use of minimum-sized batteries and will generate a minimum of heat so that a cooling fan will not be needed.